1. Field of the Invention
The present invention relates to methods and devices for terminating or arresting tachycardia, and specifically to such devices for emitting a stimulation pulse to the heart synchronized to heartbeats with a variable time delay which is quickly selected as the optimum delay for terminating tachycardia.
2. Description of the Prior Art
Tachycardia is the condition of an unnaturally elevated heartbeat frequency, for example, a heartbeat frequency of more than 180 beats per minute. It is known that this rapid heartbeat rhythm can be arrested by electrical stimulation pulses supplied to the heart. Such stimulation pulses must, however, be supplied to the heart within a critical time interval after a heartbeat, a so-called window. The window may be immediately after the refractory period of the heart following a heartbeat, or may be directly before a following heartbeat. It is further known that the width of the window and its location within a heartbeat cycle varies with body position, physical activities, drugs and other conditions. The windows respectively associated with such conditions or combinations of conditions may vary to such extent that the windows do not overlap, and it is therefore not possible to determine a fixed time for emitting a stimulation pulse in synchronization with the patient's heartbeat which is accurate for all conditions.
A device for terminating tachycardia having an input means responsive to heartbeat signals and a variable delay pulse circuit for supplying a stimulation pulse after a predetermined delay from one output pulse to the next by means of a feedback connection is described in U.S. Pat. No. 3,942,534. This device divides the total time interval between the termination of the refractory period and the next successive heartbeat into a grid of equidistant steps. The first stimulation pulse coincides with the end of the refractory time, the next stimulation pulse occurs, for example, ten milliseconds later, the next pulse occurs after another ten milliseconds, etc. Because the windows are frequently very narrow, the spacing between individual stimulation pulses in this conventional device must be selected correspondingly small. This, however, means that a large number of different pulses are required in order to suitably cover the entire possible interval. This results in the disadvantage that, under certain conditions, a long interval may elapse until a stimulation pulse is emitted during the critical time interval and tachycardia is arrested. Tests undertaken with devices functioning in accordance with the teachings of U.S. Pat. No. 3,942,534 have shown that under worst-case conditions, up to 15 minutes may elapse between the beginning of tachycardia and its subsequent arrest.
Further tests have shown that the ease with which tachycardia may be terminated increases as the time required to supply a stimulation pulse within the window decreases. This is particularly true for tachycardia which affect the atrioventricular node.